Producing metallic hydrogen has been a great challenge in condensed matter physics.

Metallic hydrogen may be a room-temperature superconductor and metastable
when the pressure is released and could have an important impact on energy and
rocketry. We have studied solid molecular hydrogen under pressure at low
temperatures. At a pressure of 495 gigapascals, hydrogen becomes metallic, with
reflectivity as high as 0.91. We fit the reflectance using a Drude free-electron
model to determine the plasma frequency of 32.5 ± 2.1 electron volts at a temperature
of 5.5 kelvin, with a corresponding electron carrier density of 7.7 ± 1.1 × 1023 particles
per cubic centimeter, which is consistent with theoretical estimates of the
atomic density. The properties are those of an atomic metal. We have produced
the Wigner-Huntington dissociative transition to atomic metallic hydrogen in
the laboratory.

Several key problems in physics involving hydrogen include production of the metal- lic phase, high-temperature superconduc- tivity, and controlled nuclear fusion (1). The transition to solid metallic hydrogen
(SMH) was envisioned by Wigner and Huntington
(WH) more than 80 years ago (2). They predicted
a first-order dissociative transition to an atomic
lattice through compression of solid molecular
hydrogen to a sufficiently high density. Solid
atomic hydrogen would be a metal with one
electron per atom with a half-filled conduction
band. Although WH’s density for the transition
was approximately correct, their predicted pressure of 25 GPa (100 GPa = 1 megabar) was way
off because they incorrectly used the zero-pressure
compressibility for all pressures. Wigner and
Huntington predicted a simple phase diagram.
Enormous experimental and theoretical developments dramatically reshaped the phase diagram of hydrogen (Fig. 1) over the past decades.
Modern quantum Monte-Carlo methods and
density functional theory predict pressures of
~400 to 500 GPa for the transition (3–5), with
an atomic lattice being in the I41/amd space
group (5, 6). Metallic hydrogen (MH) may be a
high-temperature superconductor, predicted by
Ashcroft (7), with critical temperatures possibly
higher than room temperature (8, 9). Moreover,
other predictions suggest SMH is metastable at
room temperature when the pressure is released
(10). The combination of these expected properties makes SMH important for solving energy
problems and can potentially revolutionize rocketry as a powerful propellant (11).

The pathways to MH require either increasing
pressure at low temperature (Fig. 1, pathway I)
or increasing temperature to cross the plasma
phase transition (Fig. 1, pathway II) (12–17). Pathway I transitions through a number of phases
not envisioned in the simple phase diagram predicted by WH. The low-pressure properties of solid
molecular hydrogen are fascinating, and many
aspects—such as the importance of ortho-para
concentrations, and solid-solid phase transitions
characterized by orientational order—have been
reviewed elsewhere (3, 18). In the low-pressure,
low-temperature phase I, molecules are in spherically symmetric quantum states and form a hexagonal close-packed structure. Phases II, III, and
IV are phases with structural changes and orientational order of the molecules (19–23). A new phase
in hydrogen observed at liquid-helium temperatures believed to precede the metallic phase was
called H2-PRE (24) [also named VI at higher
temperatures (25)].

We carried out a rigorous strategy to achieve
the higher pressures needed to transform hydrogen to SMH in a diamond anvil cell (DAC).
Diamond failure is the principal limitation for
achieving the required pressures to observe SMH.
We believe that one point of failure of diamonds
arises from microscopic surface defects created
in the polishing process. We used type IIac conic
synthetic diamonds (supplied by Almax-Easylab)
with ~30-mm-diameter culet flats. We etched off
~5 mm from the diamond culets using reactive
ion etching to remove surface defects (figs. S7
and S8) (26). We vacuum annealed the diamonds
at high temperature to remove residual stresses.

A second point of failure is diamond embrit-tlement from hydrogen diffusion. Hydrogen candisperse into the confining gasket or the dia-monds (at high pressure or temperature). Asan activated process, diffusion is suppressed atlow temperatures. We maintained the sample atliquid-nitrogen or liquid-helium temperaturesduring the experimental runs. Alumina is alsoknown to act as a diffusion barrier against hy-drogen. We coated the diamonds along with themounted rhenium (Re) gasket with a 50-nm-thicklayer of amorphous alumina through the pro-cess of atomic layer deposition. We have foundthrough our extensive experience with aluminacoatings at high pressures that it does not af-fect or contaminate the sample, even at tem-peratures as high as ~2000 K (12). Last, focusedlaser beams, even at low laser power (10 mW)on samples at high pressures in DACs, can alsolead to failure of the highly stressed diamonds.Laser light in the blue spectral region appears tobe particularly hazardous because it potentiallyinduces the growth of defects (27). Thermal shockto the stressed culet region from inadvertent laserheating is another risk. Moreover, a sufficientlyintense laser beam, even at infrared (IR) wave-lengths, can graphitize the surface of the dia-mond. Thus, we studied the sample mainly withvery low-power, incoherent IR radiation from athermal source and minimized illumination ofthe sample with lasers when the sample was atvery high pressures.

We cryogenically loaded the sample chamber at 15 K, which included a ruby grain for
pressure determination. We initially determined
a pressure of ~88 GPa by means of ruby fluorescence (26). Determining the pressure in the
megabar regime is more challenging (26). We
measured the IR vibron absorption peaks of
hydrogen at higher pressures (>135 GPa) with
a Fourier transform IR spectrometer with a
thermal IR source, using the known pressure
dependence of the IR vibron peaks for pressure
determination (26). We did this to a pressure of
~335 GPa, while the sample was still transparent (Fig. 2A). The shift of the laser-excited
Raman active phonon of the diamond in the
highly stressed culet region is currently the
method used for determining pressure at extreme high pressures. For fear of diamond failure
due to laser illumination and possible heating
of the black sample, we only measured the
Raman active phonon at the very highest pressure of the experiment (495 GPa) after the
sample transformed to metallic hydrogen and
reflectance measurements had been made. We
equipped our DACS with strain gauges that allowed us to measure the applied load, which we
found was proportional to pressure during calibration runs (26). We estimated pressure between 335 and 495 GPa using this calibration.
We increased the load (pressure) by rotating a
screw with a long stainless-steel tube attached
to the DAC in the cryostat. Increasing the pressure by rotating the screw after the 335-GPa
pressure point resulted in our sample starting
to turn black (Fig. 2B) as it transitioned into the
H2-PRE phase (24). Earlier studies of hydrogen
reported the sample as black at lower pressures
(28), but we believe that this is a result of different pressure calibrations in this high-pressure
region (26).